[quote]Dont you think 1mm is a bit small for a valve increase on these motors? Dont you use the stock seat at just 1mm oversized? When I got looking into the head I noticed that the exh valve seat was quiet a bit smaller than the bowl but that was not the case on the intake side. I was thinking that increasing the seat size to mach the bowl would be better but that would take 2-3mm over sized valve right?
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>>>*You are correct in that larger will add even more airflow. Increases in flow at the exhaust size work best at optimum, so one has to take into account all of the other variables such as swept area(volume) and expected normal RPM powerband.

It's always a guess for the most part.

One thing is sure, volume area on the exhaust side increases RPM potential. So if the valve and port gets too large, the engine will go lazy at slower speeds.

Taking 1mm out of the inside dimension of a valve seat adds a LOT of area. If the valve seat is already too restrictive, then the gains are huge. If the seat and port are not highly restrictive, the gains now become small.

The idea is to hit a balance that maintains throttle response and gets one closer to that perfect combination needed for nice throttle response and power.

Now we know that a 1mm oversize intake adds more area than a 1mm oversize exhaust. The balance stays pretty close.

When we tested valves on the little 22R engine way back, we found a full 1mm (.039") exhaust valve was too big. But only when we wanted to maintain low end torque and response.
So we hit on .024" as perfect.

Now I get laughed at from time to time by folks that tell me that .024" bigger won't do anything....Think of it like this: That is surface area, .024" wide times 1.471"(roughly) times 3.1416. Quick, somebody do the math, what's that in square inches compared to the entire valve head size stock in square inches, don't forget to subtract the 8mm stem surface area.

*Ran out of fingers...*LOL** (There will be a test at the end)


You could be right on the need for an even larger exhaust, it's just my feelings right now that this isn't correct for this engine design. I think one could lose low end power very easily by going too large, especially adding headers and a free flowing exhaust.

Note the "I think" part, still experimenting.

Now if there is a Turbo involved, or high RPM use is desired, that all went out the window, and we start over.

A quick example one can use is the common 350 Chev. We use 1.94-1.50" valves for stock, and 2.02"-1.60" for performance. 2.05" valves are available, nothing happens. 1.65" exhaust are available, nothing happens. Now when the rules say 2bbl carb and stock exhaust, we use 1.85" valves and 1.50" exhaust for a best choice, and I have even cut exhaust valves down to 1.460" and installed a seat to keep the velocity up on highly restricted engines..Everyone told me I was nuts, but we got the track record AND the Championship.....The key is balance, hit it and the engine is sweet....*EB